[MINC-users] register (and tagtoxfm) can produce reflections in the output transformations

[Andrew Janke]

Hi Matthijs,

This is expected behaviour. I often use register and tag points to
resolve rotation/inversion issues in data.

Thanks

a


On 5 October 2016 at 05:50, Matthijs van Eede
<matthijs.vaneede at sickkids.ca> wrote:
> Greetings,
>
> We were trying to align two brains and found that using register it's possible to get two kinds of transforms. If you pick the right tag points, you get a proper transformation file. If you happen to flip left and right in one of the tag points, a transformation is produced that flips one of the axes. The question is whether this is expected behaviour for register. Flipping axes is rarely something you want when estimating a transformation (In this case, the user that had produced these transformations with register ran a registration pipeline where all brains were left-right flipped without realizing it). Should there be an additional check in register to ensure that the determinant of a rotation matrix is positive?
>
> Here's an example of tag points that produces a transformation with a negative determinant:
>
> ######################
> MNI Tag Point File
> Volumes = 2;
> %VIO_Volume: model.mnc
> %VIO_Volume: source.mnc
>
> Points =
>  -0.366219818592072 7.60433340072632 2.30845427513123 1.40657377243042 -4.23010206222534 17.1643028259277 ""
>  0.120084546506405 -0.405666679143906 2.30845427513123 0.545454323291779 -4.9874153137207 25.2130146026611 ""
>  0.174118369817734 -5.15316677093506 1.66004836559296 0.592638909816742 -4.23246097564697 30.2041149139404 ""
>  -3.39211368560791 -3.08416676521301 -0.879541099071503 -3.08776211738586 -1.92041432857513 28.3605556488037 "";
> ######################
>
> Cheers
> Matthijs & Ben
>
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